The invention relates to the measuring of distance between an active installation, which will be called hereafter "hunter" and a member passive in the measurement, which will be called hereafter "target". It finds a particularly important, although not exclusive, application for measuring the distance and position of the target with respect to the hunter, particularly for allowing the meeting and mooring of two space vehicles.
In the latter, case, it is necessary for the hunter to be provided with a device for:
measuring the distance of the target and its position with respect to the hunter, as soon as the distance separating them is less than about 100 km, PA1 also determining the attitude of the target, but solely for much shorter distances, not exceeding a few tens of meters.
Measuring devices have already been proposed for use for this purpose including, on the target, several reflectors spaced apart in a given geometrical pattern and, on the hunter, a monochromatic light pulse source and a matrix detector for forming an image of the reflectors. The detector is generally formed by a matrix of charge coupled sensors or CCD. The attitude of the target with respect to the hunter is then determined by comparing the image obtained at a short distance with the known distribution pattern of the reflectors. At a short distance, it is also possible to measure the distance separating the hunter and the target by analysis of the image, but, as soon as the distance becomes great, it can no longer be determined by measuring the size of the target for this image is practically a pin point for the detector. In this case, it is known to determine the distance by measuring the flight time of a pulse delivered by the light source. The flight time is in effect equal to twice the distance divided by the speed of light.
But, when the distances are great, the power which returns to the hunter is very low and the problem arises of identifying the echo in the image, where parasites appear due to sources situated in the field, particularly when the sun is situated therein.
This problem is all the more serious since the conventional method of using the CCD includes a phase of accumulation of the charges generated in each of the sensors by the incident photons. The permanent sources, such as the sun, create charges during the whole duration of the integration phase, very much greater than the duration of the pulses delivered by the source.
It has already been proposed to overcome this problem by interposing, in the return path of the light echo to the CCD, a mechanical or optoelectronic shutter having an opening time almost equal to the time of the pulse delivered by the source. By moidifying the delay in opening of the shutter with respect to the emission of the pulse, the moment is sought when a maximum output signal is obtained. From the delay, which corresponds to the flight time, the distance may be derived.
This solution is unsatisfactory. The mechanical or optoelectronic shutter is expensive. Its use is complex. The measurement involves proceeding by trial and error so as to obtain coincidence between the return time of the pulse and the opening of the window, that is to say synchronizaton.
The invention aims at providing a measuring method and device answering better than those known heretofor the requirements of practice, particularly in that they allow distance measurements to be made without adding any mechanical or electromechanical member to the means which are in any case required for position and/or attitude measurement.